Polyether polyols which are suitable for the preparation of polyurethane materials can be obtained via various preparation processes. On the one hand, the base-catalyzed addition of alkylene oxides to starter compounds having Zerewitinoff-active hydrogen atoms and on the other hand, the use of double metal cyanide compounds as catalysts for the addition of alkylene oxides to starter compounds having Zerewitinoff-active hydrogen atoms are of importance on the large industrial scale. The (Lewis) acid-catalyzed addition of alkylene oxides on to suitable starter compounds is of minor importance.
The base-catalyzed addition of alkylene oxides, such as, for example, ethylene oxide or propylene oxide, to starter compounds having Zerewitinoff-active hydrogen atoms is carried out in the presence of alkali metal hydroxides, alkali metal hydrides or also amines, such as N,N-dimethylbenzylamine or imidazole. After the addition of the alkylene oxides has taken place, the polymerization-active centers on the polyether chains must be neutralized with dilute mineral acids, such as sulfuric acid or phosphoric acid, or also organic acids, such as e.g. lactic acid, and the salts formed must be separated off, if appropriate. Working up of the alkaline polymers is also possible with the aid of acid laminar silicates or by means of acid cation exchangers. In amine-catalyzed alkylene oxide addition reactions, further working up can be omitted if the presence of the amines in these polyether polyols does not impair the preparation of polyurethane materials. In addition to the necessity of the polyether polyols having to be worked up, if appropriate, before their use as polyurethane components, two further disadvantages of the base-catalyzed addition of alkylene oxides on to starter compounds having Zerewitinoff-active hydrogen atoms are to be mentioned. Only polyethers having relatively low equivalent weights can be obtained via amine catalysis, in this context see, for example, Ionescu et al. in “Advances in Urethane Science & Technology”, 1998, 14, p. 151-218. Under alkali metal hydroxide catalysis, undesirable side reactions increase significantly as the molar mass of the polymer increases. The isomerization of propylene oxide to allyl alcohol, which, at high equivalent weights (or low OH numbers), leads to a high content of monofunctional polyether species in the reaction mixture, is to be mentioned in particular here. The monofunctional polyether molecules have an adverse effect on the full curing properties and the profile of physical properties of polyurethane systems.
By employing double metal cyanide catalysts, it has become possible to speed up the addition of alkylene oxides, in particular propylene oxide, to starter compounds having Zerewitinoff-active hydrogen atoms down to very low OH numbers, without the abovementioned isomerization of propylene oxide to allyl alcohol occurring to a noticeable extent. Highly active DMC catalysts, which are described e.g. in U.S. Pat. No. 5,470,813, EP-A 700 949, EP-A 743 093, EP-A 761 708, WO 97/40086, WO 98/16310 and WO 00/47649, furthermore have an exceptionally high activity and render possible the preparation of polyether polyols at very low catalyst concentrations (25 ppm or less), so that it is no longer necessary to separate off the catalyst from the finished product. Typical examples are the highly active DMC catalysts described in EP-A 700 949, which, in addition to a double metal cyanide compound (e.g. zinc hexacyanocobaltate(III)) and an organic complexing ligand (e.g. tert-butanol), also contain a polyether having a number-average molecular weight of greater than 500 g/mol.
A characteristic of DMC catalysts is their pronounced sensitivity to high concentrations of hydroxyl groups, which can be caused, for example, by large amounts of starter compounds such as ethylene glycol, propylene glycol, glycerol, trimethylolpropane, sorbitol or sucrose, and polar impurities of the reaction mixture. The DMC catalysts then cannot be converted into the polymerization-active form during the reaction initiation phase. Impurities can be, for example, water, compounds having a high number of hydroxyl groups in close proximity, such as carbohydrates and carbohydrate derivatives, or compounds having basic groups, such as, for example, amines. Substances having carbonyl groups in close proximity or carbonyl groups adjacent to hydroxyl groups also have an adverse effect on the catalyst activity. In order, nevertheless, to be able to subject starter compounds having high concentrations of OH groups or starter compounds with impurities which are to be regarded as catalyst poisons to DMC-catalyzed alkylene oxide addition reactions, the concentration of hydroxyl groups must be lowered or the catalyst poisons rendered harmless. For this purpose, in the past prepolymers were first prepared from these starter compounds by means of base catalysis, and, after thorough working up, it was then possible to convert these into the desired alkylene oxide addition products of high molar mass by means of DMC catalysis. An important further development in this connection was the development of continuous metering of starter compounds, which is disclosed in WO 97/29146. In this case critical starter compounds are not initially introduced into the reactor, but are fed to the reactor continuously during the reaction, alongside the alkylene oxides. In this process, prepolymers can be initially introduced into the reactor as the starting medium for the reaction, and the use of small amounts of the product to be prepared itself as the starting medium is also possible. The necessity of having first to prepare prepolymers suitable for further alkylene oxide additions separately was eliminated with the latter procedure.
However, if short-chain polyether alcohols having OH numbers of greater than 200 mg KOH/g are to be obtained, it is necessary to increase the ratio of starter compound to alkylene oxide in the educt stream metered in, so that there is again the danger of reaching critical concentrations of hydroxyl groups and polar impurities. In such cases the catalysts increasingly lose activity during the starter compound metering phase, which manifests itself e.g. by an increase in pressure in the reactor as a consequence of an increasing concentration of free alkylene oxide.
If compounds of varying purity, for example those which are obtained from renewable sources of raw materials, are employed as starter compounds, unknown secondary components can likewise significantly impair the catalyst activity. In this case either the catalyst cannot be converted into the active form at all, or the loss in activity described above is observed.
The problems described on the one hand, can of course be counteracted by an increase in the catalyst concentration, and on the other hand an attempt can also be made to free the starter compounds from impurities by suitable pretreatments, such as intensive stripping at temperatures above 80° C., distillation or extraction. All these alternatives are time-consuming and cost-intensive.